The present invention relates to a method of manufacturing a rock bit cone rotatably supported by each of a plurality of bearing pins extending centripetally obliquely equiangularly from a rock bit body and having a number of teeth on a conical outer surface thereof.
Petroleum and natural gas exist, generally beneath a cap rock. Therefore, in order to prospect for them and to mine them, it is necessary to drill a rock layer by using a drilling facility provided on the ground or sea surface.
As the rock bit for drilling rock layer, blade bit, cone bit and diamond bit etc. have been known. Among others, the cone bit has been widely used.
The conventional cone bit comprises a rock bit body formed in an upper portion thereof with a thread into which a drill collar of a drill pipe is screwed, a plurality of equiangularly spaced bearing pins extending centripetally obliquely from an inner face of a leg portion formed in a lower portion thereof. Each bearing pin supports rotatably a cutter in the form of a cone having a conical outer surface in which a number of teeth are implanted.
In drilling a rock layer, a drill collar mounted on a lower end of a drill pipe is screwed onto the threaded portion of the bit body and the drill pipe is rotated by a rotary table of a drilling rig arranged on the ground or sea surface, so that the cones are rotated around the bearing pins by means of contacts between the teeth thereof and the rock layer. Thus, portions of the rock layer are crushed, turned up and kicked out by the teeth. On the other hand, high pressure mud is supplied through the drill pipe to the cone bit by a mud pump provided in the drilling rig. The high pressure mud functions to lubricate the teeth of the cones and carry the crushed rock portions through an annular space formed between an outer surface of the drill pipe and a wall of a drilled hole up to the surface of ground or sea.
Therefore, the teeth must be of highly hard material. A TCI (tungsten carbide insert) bit having implanted inserts each of tungsten carbide or a milled tooth bit having teeth each prepared by machining and then hard-facing the surface thereof with a hard metal has been used conventionally. The TCI bit is usually manufactured by forming a cone body by forging, boring holes in places of a surface thereof, in which cylindrical inserts are to be implanted, by a boring machine and pressing these inserts into the respective holes. Therefore, it requires a number of manhours and it becomes very expensive, necessarily. On the other hand, the hard-facing technique which is necessary to manufacture the milled tooth bit usually contains some uncertainty and it is very difficult to obtain a uniform hard metal layer on the teeth. Even if a uniform layer is provided, it is usually peeled off easily by mechanical shock. Further, the milled tooth bit is also expensive.